In recent times such small internal combustion engines have been subjected to increasingly stringent standards, particularly in relation to pollution. Such standards are having a profound influence on engine design and it is becoming increasingly difficult for the necessary performance standards specified in the environmental regulations to be met by small two stroke engines, small four stroke engines, and hybrid two/four stroke engines.
In order to meet the increasingly stringent standards, it is necessary for virtually every aspect of the internal combustion engine to be optimised. In particular, many aspects of the engine performance, including the use of exhaust catalytic mufflers, stratified combustion, and the like depend upon the rigorous control of the fuel to air ratio which must be maintained within a narrow operating range.
However, the fuel to air ratio can be significantly changed by the quality and cleanliness of the air cleaner used to filter the pre-combustion air before it enters the engine.
There are, in general, two types of air cleaners which are extensively used and these can be classified as either “wet” or “dry”. Wet air filters are porous plastic foam, or similar, impregnated with a viscous liquid such as oil. Dry air filters use porous plastic foam, porous felt, a porous paper cartridge, or some other porous material. Both these type of filters require routine maintenance and, if not properly maintained, can significantly alter the fuel/air ratio. For example, if the “wet” filters have an excess of oil applied thereto, this can increase the particulate material which is present in the exhaust. If the dry types of filter become blocked to at least an appreciable extent with foreign matter such as dust, grass cuttings or the like, this can change the ratio of the fuel air mixture thereby resulting in both increased fuel consumption and an increased pollution level. Further, where a muffler catalytic converter is present, a fuel rich mixture of fuel and air can lower the operating temperature of the catalytic converter from its intended high operating temperature thereby making the catalytic converter less effective, and possibly permanently disabling the converter.
U.S. Pat. No. 3,855,976 granted to the present applicants discloses a substantially self-cleaning air filter system in which air from the fan powered by the engine was directed to pass over a fine mesh screen. The screen both had a large surface area and was substantially perpendicular to the axis of rotation of the fan. The advantage of such an arrangement is that it meant that the engine operator was not obliged to laboriously clean the air filter at frequent specified operational periods (eg every 10 hours of operation). Instead this prior art air filter needed only relatively infrequent cleaning.
However, pollution performance standards are not the only criterion required to be met by modern day small size and lightweight engines for hand held use. For such devices, the engine attached to the device is generally the most prominent part of the device. However, conventional air filters, including the one described in the abovementioned US Patent, are relatively bulky and add to the weight and size of the overall engine unit. Since such air filters can significantly add to one or more of the dimensions of the overall engine unit, they can make the appliance far more awkward to hold, and thus more awkward to use, by the operator. Another disadvantage of this extra bulk is that it adds to the cubic capacity, and one or more dimensions of, cartons in which the engines are shipped. This increases the contribution to the final price made by the cost of transport.